In numerous situations, when explosive devices are prepared, transported, or otherwise handled, certain amounts of the explosive material end up on surfaces. Such surfaces may be clothing, a container, a vehicle, the ground, window sills, and so on. Failure to detect such materials on everyday items can result in concealed assembly and illegal transport of explosive materials and devices.
Explosives (unlike most other materials) generally are composed of a fuel and an oxidizer component. These will react under appropriate conditions (e.g., the addition of energy via heating or other means). An explosion requires that the combustion reaction occur at a rate such that shock waves are produced. Under alternative conditions, the reaction rate may be such that a release of the internal energy of the explosive, such as by combustion (oxidation of the fuel), occurs, but with no shock wave or explosion. In either case, the release of internal energy will have a measurable acoustic signature that can be used for detection.
Many detection methods have been used to detect explosives. Low intensity lasers have been used for photoacoustic spectroscopy (PAS), which detects a very weak acoustic signal caused by laser-induced sample heating. The heating and resultant acoustic signal are proportional to the material's absorption of energy. PAS is generally used to determine a material's absorption of energy as a function of laser wavelength, by identifying an explosive material from a comparison of the material's absorption of energy at a wavelength at which the explosive material is known to absorb energy, compared to the material's absorption of energy at a non-absorbing wavelength. PAS has had limited success in detecting explosives in realistic environments, because explosives lack sufficiently distinct absorption characteristics for low false alarm detection. PAS also requires probing a material with at least two laser wavelengths, as discussed above.
Most other explosive detection techniques use auxiliary properties (chemical or optical) of the explosives or their components for detection. For example, Raman-based detection detects scattered light whose wavelength shifts are related to the vibrational structure of the explosive molecules. One potential pitfall of such techniques is that similar properties (e.g., vibrational structure) may exist in other non-explosive materials, giving rise to false alarms. For example, X-ray transmission, X-ray backscatter, and THz imaging are sensitive only to bulk amounts of explosive materials or to metallic constituents in explosive devices. Ion-mobility spectrometry (IMS) requires surface sampling, for instance by airflow agitation, followed by collection of dislodged particles. Thus, the detection is relatively slow, and it is effective only at short distances (e.g., in a range of substantially less than about 1 meter). Raman spectroscopy has a very weak signature, requiring data collection for an extended period of time. Laser-induced breakdown spectroscopy (LIBS) is prone to generating false alarms in many situations, because it is largely non-specific, as it detects atomic constituents which are found in many compounds (oxygen and nitrogen). Differential reflectometry is effective only from relatively short distances (e.g., about 1 meter), and it is also prone to generating false alarms, because the signature that it relies on is complex and not well defined. Fluorescence quenching (e.g., the Fido™ detector by ICx Technologies, Arlington Va.) has some of the drawbacks of IMS discussed above: it requires that the molecules to be detected reach the detecting device in order to interact with a fluorescing polymer. The technique is therefore limited to stand-off distances in a range of less than about 1 meter.
Therefore, there is a need for a method of detecting explosives at a distance that minimizes or eliminates the above mentioned problems.